On a general point I can see why some people can't deal with the paradox of people having confidence in a design yet still calling it high risk at the same time.

At heart it's the difference between science and engineering. It's also about the credibility of a team working on something versus a knowledgeable (in the subject) team (but not working on that particular problem) saying the (first) team can do something.

Science searches for the detail rules that build the universe. Engineering uses the rules that have been found to solve specific problems.

A scientist who can only explain 1/2 of why something works as it does is a very unhappy person. An engineer would ask "does that that explain enough of the phenomena involved to let me build what I want?"

What the ESTEC team and the symposium attendees agreed on is that REL have a) A clear path from what they have built to what they want to build and b) that it involves no new science. Existing models and design tools are good enough to build the vehicle with a high degree of confidence that it will work.

A classic contrast would be with the X30 programme (I could suggest others), where the PI asserted that the design tools of the time were good enough to build it with no independent confirmation. Likewise that the science was well enough understood to design a fuel injector that would mix, ignite and burn the fuel in the time it took to flow over the under surface of the vehicle.

Neither of these assertions was true (along with various others, as Heppleheimers book describes). The result was like trying to do brain surgery with a razor sharp but very floppy scalpel. Once an independent inspection of the NASP programme was done it was effectively dead.

People who understand engineering and who've dug into REL's programme (as the ESTEC team did, including full disclosure of the then secret frost control system) feel the science is understood well enough by REL that the engine and the vehicle will do what they say it will do.

The "high risk" part of comes from the difference between what can be done (IE does not violate chemical or physical laws) and making it work.

Leonard De Vici knew that better, cheaper bearings would have a major effect on how well machinery ran IE he understood the science well enough to recognize there benefits. But he did not have the engineering to make them good enough on a big enough scale (and hence cheap enough for mass use) to deliver any ideas he might have had for using them.

For SABRESkylon the risk starts with wheather they can raise all the money the cost models say you need for the project. This is the biggest risk since inadequate funding reduces margins for coping with unknown problems.

SABRE and Skylon are very large systems with lots of parts and lots of interactions between the parts. Can they be made at the price, quality and volume levels needed? Can they be assembled at the rate needed to deliver a vehicle before the money runs out? Will testing reveal unexpected interactions between systems (probably)? If harmful problems are found can work arounds be found?Will the work arounds so damage the capabilities of the vehicle to make it unsaleable or economically unusable?

The open ended nature of some of those risks is what makes the SABRESkylon programme "high risk"

If some of the up-front risks materialize the programme never starts (or cripple it critically later on) while some of the later ones could kill it right at the last hurdle and you will only know you've succeeded when the last Skylon needed to break even has been sold.

I hope that explains how people can both be confident that SABRESkylon can work (IE no new science and no impossible physics involved) but also be conscious that there is still a very large degree of risk in making it work.

The payoff. Countries, corporations or even (very) wealthy individuals can acquire on-demand launch of their payloads (or themselves if they are happy to take the risk) with a safety record orders of magnitude above what they could ever develop for themselves of up to 15tonnes to LEO, more than 5 tonnes to GTO with a reusable tug and several tonnes to escape velocity (letting you run your own Moon/Mars/Venus/Wherever exploration programme) , and recover a chunk of their investment if they run out of payloads to launch by simply selling it to someone else.

It's a rule that VC funders invest in the team, not the details of an idea. To date the REL team has shown that when fully funded they have delivered what they promised roughly when they promised it and coped with any issues along the way. That includes a great many other side projects run with other partners to develop and verify various SABRE specific parts. Those projects only appear as single slides in their summary presentations but demonstrate that REL has run a great many more projects than most people realize. The SABRE ground demonstration will not be their first trip round the block by a very long way.

But it does seem Space X gets a free pass from criticism on here by some yet REL gets told that will never happen, perhaps some tire of the double standards expressed.

The "double standard" is due to one group actually building and flying real hardware, whereas the other group only has managed to build test hardware of small components, all the while claiming that there "are no showstoppers" for building something much better.

But it does seem Space X gets a free pass from criticism on here by some yet REL gets told that will never happen, perhaps some tire of the double standards expressed.

The "double standard" is due to one group actually building and flying real hardware, whereas the other group only has managed to build test hardware of small components, all the while claiming that there "are no showstoppers" for building something much better.

Which group do you think would naturally be taken more seriously?

That's a far from fair comment being as REL have had to traverse a literal labyrinth to obtain the financing they have so far unlike Space X.

But it does seem Space X gets a free pass from criticism on here by some yet REL gets told that will never happen, perhaps some tire of the double standards expressed.

The "double standard" is due to one group actually building and flying real hardware, whereas the other group only has managed to build test hardware of small components, all the while claiming that there "are no showstoppers" for building something much better.

Which group do you think would naturally be taken more seriously?

That's a far from fair comment being as REL have had to traverse a literal labyrinth to obtain the financing they have so far unlike Space X.

It may not be "fair", but it is accurate. Fairness has nothing to do with it. And SpaceX has not had an "easy street" either.

But it does seem Space X gets a free pass from criticism on here by some yet REL gets told that will never happen, perhaps some tire of the double standards expressed.

The "double standard" is due to one group actually building and flying real hardware, whereas the other group only has managed to build test hardware of small components, all the while claiming that there "are no showstoppers" for building something much better.

Which group do you think would naturally be taken more seriously?

That's a far from fair comment being as REL have had to traverse a literal labyrinth to obtain the financing they have so far unlike Space X.

It may not be "fair", but it is accurate. Fairness has nothing to do with it. And SpaceX has not had an "easy street" either.

As for confidence. We know 3 vehicles have demonstrated winged reentry from full LEO velocity.

And that's the sort of exaggeration that makes everything else you say suspect. There's no analogous relationship between the Skylon design and the two round-nosed, stubby-body, delta-wing orbiters (STS/Buran), nor with the tiny little X-37 spaceplane.

Skylon is an entirely new untried dissimilar design. You can't just say "Oo, look, wings" and expect anyone to take you seriously.

(As Lars-J notes, it'd be like comparing a reusable upper-stage and a capsule, "Oo, look, wingless". There's just no comparison. And Lars could have also added a re-entry body for a warhead. Or even those three winged re-entry vehicles: They have as much similarity, from a physics standpoint, to a reusable upper-stage as they have to Skylon. (Ie, none.))

New material, new engines, smaller wings than ever before (while still functioning as an airplane),

Wing loading for the MD MD11F was 844 Kg/m^2. That was for a passenger carrying airliner. I don't have a figure for wing area for Skylon but anything above 386 m^2 would put it's wing loading below that.

With a length of 82 meters, and the design on wikipedia remotely to scale, it's considerably less, indeed. That's the point of a SSTO. Smaller wings, less mass to haul along. Which means a higher cruising speed to compensate for the smaller wings.

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dedicated landing strips,

Wrong again. A dedicated launch runway to orbit, but in air breathing only mode Skylon can take off from a much wider range of runways. Most of the length is to meet emergency stop criteria for the fully loaded vehicle, which only apply when it's fully loaded to go to orbit. Landing is much easier with a low empty weight and no engine noise issues.

That's what I said. The Skylon needs a dedicated runway to get to orbit. Using the SABRE engine for an airplane doesn't require the Skylon design.

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'detachable' carbo bay that can be preloaded and quickly swapped upon landing...

Much like how most passenger luggage is shipped in fact.

Oh yeah, last time I took the airplane, the preloaded cargo bay was inserted into the plane. Totally forgot about that one. And after that, the passenger cabin was attached to the wings that had just landed. No reason to wait to board until the previous passengers had gotten off.

Wait, that design also exists only on the drawing board. (I've tried finding the thread, but no such luck. Apparently, I didn't post in it).

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On a general point I can see why some people can't deal with the paradox of people having confidence in a design yet still calling it high risk at the same time.

Curious statement, coming from the man who's arguing that there's nothing that revolutionary about the Skylon. Personally, I think it has quite a few new features, that'll require a lot of funding to get the kinks out. It's high risk, with possible high gain (as you said yourself, very confusing compared to the post I quoted above). Will it fly? Who knows. Personally, I'm not going into that one until there's a full scale model of the SABRE engine. Otherwise, there's nothing but opinion to argue with.

New material, new engines, smaller wings than ever before (while still functioning as an airplane),

Wing loading for the MD MD11F was 844 Kg/m^2. That was for a passenger carrying airliner. I don't have a figure for wing area for Skylon but anything above 386 m^2 would put it's wing loading below that.

With a length of 82 meters, and the design on wikipedia remotely to scale, it's considerably less, indeed. That's the point of a SSTO. Smaller wings, less mass to haul along. Which means a higher cruising speed to compensate for the smaller wings.

I don't think comparing wing loading calculated makes much sense when comparing skylon against traditional airliners; On Skylon, most of the lift comes from the body, not from the wings. So the "effective wing loading" of Skylon may be less than half of the "calculated wing loading" of Skylon.

And that's the sort of exaggeration that makes everything else you say suspect.

And that's the sort of pointless comment that makes people suspect you have another agenda.

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There's no analogous relationship between the Skylon design and the two round-nosed, stubby-body, delta-wing orbiters (STS/Buran), nor with the tiny little X-37 spaceplane.

I'll spell it out for you. There are 2 proven full size (for their task) configurations which have been shown to survive return from orbit. Stubby, dense capsules and wing/fuselage designs. If Dream Chaser flies we can add lifting body as well.

Of these Shuttle was the closest to a full stage, with main engines and payload. We know such a structure can be built and work. That's the "known" part of the equation.

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Skylon is an entirely new untried dissimilar design.

I spent a whole post explaining my views on risk. But it seems you see what you want to see.

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You can't just say "Oo, look, wings" and expect anyone to take you seriously.

Nor did I. But again you seem to see what you want to see.

I didn't bother to emphasize that Skylon is a fully reusable vehicle and is expected to go through a full test programme like an aircraft. I thought it was redundant. Obviously an oversight on my part.

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(As Lars-J notes, it'd be like comparing a reusable upper-stage and a capsule, "Oo, look, wingless".

Which may explain why trying to land an upper stage has proved so difficult.

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There's just no comparison. And Lars could have also added a re-entry body for a warhead. Or even those three winged re-entry vehicles: They have as much similarity, from a physics standpoint, to a reusable upper-stage as they have to Skylon. (Ie, none.))

Again the specific point I was covering was re-entry. They all do (or did) it roughly horizontally. In fact they will have more similarity to such an upper stage as they all went up vertically and spent most of their re-entry moving horizontally. I await the first upper stage ever to duplicate that feat with much interest.

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Firstly, if you are going to address a comment to me, address it to me. "Some people" is just passive aggressive nonsense.

It was only when I took a step back that it occurred to me that the fact I could hold what seemed opposite views about what seemed the same thing could seem strange.

You have a very high opinion of yourself if you think my post was only addressing you.

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Secondly, you couldn't have missed my point more if you'd deliberately got into a car and travelled in the opposite direction until you hit ocean.

Perhaps you should explain exactly what was your point? I got lost in amongst the various quotes you included.

With a length of 82 meters, and the design on wikipedia remotely to scale, it's considerably less, indeed. That's the point of a SSTO. Smaller wings, less mass to haul along. Which means a higher cruising speed to compensate for the smaller wings.

Wing loading or wing area? Either way shifts you comment

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That's what I said. The Skylon needs a dedicated runway to get to orbit. Using the SABRE engine for an airplane doesn't require the Skylon design.

There's what you seem to have meant and what you wrote. The term you're looking for is "runway"

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Much like how most passenger luggage is shipped in fact.

Oh yeah, last time I took the airplane, the preloaded cargo bay was inserted into the plane. Totally forgot about that one. And after that, the passenger cabin was attached to the wings that had just landed. No reason to wait to board until the previous passengers had gotten off.

I doubt you've ever paid any attention to how aircraft passenger luggage is shipped. It mostly uses these things.

So yes actually most passenger luggage is shipped in pre loaded units, which is how I was careful to phrase it.

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Wait, that design also exists only on the drawing board. (I've tried finding the thread, but no such luck. Apparently, I didn't post in it).

And the aircraft involved has even been named on this thread, which makes your inability to find it even worse....

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Curious statement, coming from the man who's arguing that there's nothing that revolutionary about the Skylon. Personally, I think it has quite a few new features, that'll require a lot of funding to get the kinks out. It's high risk, with possible high gain (as you said yourself, very confusing compared to the post I quoted above).

Then perhaps you should try reading this post, which explains my views.[quote john smith 19 link=topic=36826.msg1565303#msg1565303 date=1470074522]

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Will it fly? Who knows. Personally, I'm not going into that one until there's a full scale model of the SABRE engine. Otherwise, there's nothing but opinion to argue with.